Guided wave optical structures/circuits are typically optical fibres and planar integrated optical waveguides.
Optical waveguides may support from zero to several bound modes of the optical field depending upon parameters such as cross-sectional geometry, cross-sectional refractive index profile and wavelength. A (bound) mode is a stable state of the optical field, which is bound to the waveguide, having a substantial amount of the optical energy confined within the core of the waveguide. As a general rule, for a waveguide that supports a plurality of modes, the fundamental mode is better bound (confined) to the waveguide core than the first higher order mode, which in turn is better bound than the second higher order mode, etc.
For a waveguide that is symmetrical (both in geometry and refractive index profile) the modes that may be bound to the waveguides have alternating even and odd field profiles, where the even and odd field profiles are mutually orthogonal, meaning that optical energy cannot be transferred between an even and an odd field profile.
Within guided wave optics it is typically of interest to have the waveguide to be single-mode, as this eliminates problems with coupling of optical energy from the fundamental mode to higher order modes. The latter can be problematic in waveguide bends where symmetry breaks down and the field profiles no longer are orthogonal. As the higher order modes may be loosely bound to the waveguide core they may radiate the coupled optical energy out into the surroundings of the waveguide core leading to loss of the optical energy. The radiation loss of optical energy in the bends is obviously furthermore dependent upon the bend radius, where a smaller (larger) radius yields larger (smaller) radiation loss.
The obvious way to circumvent this problem is to have the bent waveguide to support only the fundamental mode (even though there will be bend loss for the fundamental mode also, however, it will be much smaller), or to have a large bend radius.
The criterion for a typical waveguide to be single mode is that the cross-section is sufficiently small or the refractive index contrast between the waveguide core and the surroundings is sufficiently small. However, the smaller the cross-section/refractive index contrast the smaller the confinement of the optical field to waveguide core will be, and the larger the bend loss for the fundamental mode will be.
By increasing the bend radius the physical size of the optical waveguide circuit (be it optical fibres or planar integrated optical waveguides) increases, taking up more volume which is undesirable.
Of the two parameters cross-section vs. index contrast, the latter is the more important for good confinement meaning that it is possible to have good confinement of the optical field to the waveguide core while maintaining single-mode operation by increasing the refractive index contrast and simultaneously reducing the waveguide core cross-section. Actually, it is possible to obtain strong confinement this way, and the stronger the confinement the smaller the radius can be of waveguide bends while maintaining low bend loss.
In order to integrate different optical functions/elements in an optical circuit in a limited space the different elements must be reduced in size as to fit into the limited space. This requires small bend radii, hence waveguide structures having small cross-sectional areas and large refractive index contrasts between the waveguide core and its surroundings.
In summary: It is advantageous to use waveguides having high refractive index contrast between the waveguide core and the surroundings, as well as small core cross-sections in order to fabricate small/narrow optical structures.
For a given waveguide supporting a mode with a field profile there exists a so-called spot size which basically is a measure of the size of the field profile, and is typically denoted by the Greek letter w. In case the field profile is not circular symmetric, a spot size for the vertical as well as the horizontal dimension are assigned to the field profile. When butt-coupling together two waveguides the requirement for loss less transfer of optical energy from the first to the second waveguide is that the overlap integral between the normalized field profiles pertaining to the two waveguides is unity, which means that the two field profiles must be identical, or in other words, that the spot sizes must be identical.
Assuming coupling between two waveguides having Gaussian field distributions with spot sizes ω1 and ω2, respectively, the coupling loss (CL) in dB can be calculated from the simplified expression
  CL  =            -      20        *    log    ⁢                  ⁢                  (                              2            *                          ω              1                        *                          ω              2                                                          ω              1              2                        +                          ω              2              2                                      )            .      
For an optical circuit applying high refractive index contrast waveguides with small cross-section the spot size is necessarily small if good confinement is achieved, see e.g. John M. Senior, “Optical Fiber Communications: principle and practice”, Prentice Hall, (1985).
Optical integrated circuits are typically plugged into parts of optical fibre nets. Said nets are typically made using so-called standard fibre of which the most commonly used is the so-called SMF-28 fibre type. This kind of fibre is single mode and possesses excellent optical properties. However, it also has a relatively large spot size as it is a low refractive index contrast fibre. Hence, butt-coupling an optical waveguide circuitry made using high refractive index contrast to an SMF-28 fibre results in coupling loss—The larger the difference in spot sizes the larger the coupling loss.
For carefully tailored integrated waveguides where care has been put into design of core cross-section and refractive index difference, the coupling loss can be practically zero, whereas the coupling loss per facet for a 3 by 3 μm waveguide core, having a refractive index difference of 2.5% between core and surroundings, and an SMF-28 fibre is 3 dB, i.e., a loss of 6 dB just for connecting the waveguide circuit to the fibre net. For integrated optical circuits applying even lager refractive index differences this figure will increase, effectively disqualifying the circuits from practical use as these high loss values cannot be tolerated from a system point of view.
In summary: The problem of using integrated optical circuits with small cores and a high refractive index difference is that in coupling between said circuit and a standard fibre an intolerable high coupling loss is induced.
The solution to the aforementioned problem is to somehow shape the field profile in a section of the integrated optical circuitry just before the fibre coupling, such that the field profile from the integrated optical circuit resembles the field profile of the standard fibre, thereby reducing the coupling loss. Besides reducing the coupling loss, it must be assured that the polarization dependent loss in the coupling is maintained at a low level.
The technologies typically used for fabricating integrated optical circuits are planar, i.e., operates on planes in deposition, pattern definition and etching. This means that it is highly difficult to fabricate waveguides that varies in the vertical dimension, as the vertical dimension is defined by the thickness of the deposited core layer. It is, however, easy to vary the horizontal dimension as this can be included in the design.
Previous solutions to the problem include (periodically) segmented waveguides (cf. FIG. 2.a), where the waveguide has been segmented in such a way that the optical field is gradually lost in a manner that the field profile effectively expands thus providing better coupling to the fibre field. The basic flaw with this approach is that optical energy is actually “lost” in the spaces between the waveguide core segments, and no stable mode exists. This makes dicing and polishing precision highly critical, so that the spot size converter is terminated (attached to the fibre) just at the right position where the waveguide mode has evolved into fitting the fibre mode.
Another method uses a waveguide taper that gradually narrows the width of the waveguide (cf. FIG. 2.b). There exists an optimum width of a waveguide where the spot size of the field pertaining to the waveguide is a minimum. Increasing the width increases the spot size in the horizontal dimension while also increasing the confinement of the field. Decreasing the width also increases the spot size while decreasing the confinement of the field. If the waveguide is tapered down sufficiently, the spot size of the waveguide is increased in such a way that not only the horizontal but also the vertical dimension increases, providing good coupling to a standard fibre. However, when the waveguide core narrows the spot size becomes highly dependent upon the actual width, and since there are limited control over line widths in the fabrication process, it is difficult to reproducibly make good spot size converters this way—either the coupling is bad caused by too good confinement to the (too wide) waveguide core—or there is no optical energy to couple to the fibre since all is lost from the (too narrow) waveguide core. More ingenious solutions based on semiconductor materials and aimed at efficiently coupling a laser to an optical fibre have been proposed. EP-1 245 971 e.g. discloses a semiconductor waveguide component with a mode transition section where a tapered waveguide surrounded by two sets of lateral confinement waveguides along its tapered region and defined over a slab waveguide are provided, to reduce coupling loss to an optical fibre by matching the modal spots. WO-A1-02/29905 e.g. discloses an optical mode size converter for use between a semiconductor light source (e.g. a laser) and an optical transfer medium (e.g. an optical fibre) for receiving output light from the light source. The mode size converter comprises a lower waveguide formed on a buffer layer and an upper waveguide formed on a lower cladding layer located between the lower and upper waveguides, the upper waveguide having a branched light input unit.
A disadvantage of the latter solutions is that they require either a slab region under the waveguide or a lower and upper waveguide. They do therefore not apply to standard buried waveguide structures.
U.S. Pat. No. 6,181,860 discloses an optical waveguide with a transition portion at one end for enabling connection with an optical component (e.g. an optical fibre), the transition portion comprising a widening portion increasing in width leading to a tooth structure comprising teeth with a decreasing width.
WO-03/062883 discloses a high-index contrast waveguide coupler comprising a waveguide that is tapered down in width to an optical tip or alternatively to a combination of multiple tips.